Patent GB-605264-A (GB)
Title: Improvements in or relating to hydraulic power systems
IPC: F15B9/08
Filed: 1945-05-22
Abstract: 605,264. Fluid-pressure servomotor-control systems. BRISTOL AEROPLANE CO., Ltd., and REID, J. K. May 22, 1945, No. 12800. [Class 135] A hydraulic power system comprises a pump unit with at least two continuously driven pumps operating in closed circuits and supplying fluid to a hydraulic motor at a rate variable by a single, normally closed valve adjustable by an operator, a normally open valve being provided in each pump circuit to adjust the quantity of fluid circulating in the respective pump circuits, the normally open valve in circuit of the pump of lower or lowest capacity being closed first on movement from the normal position. The invention is applied to the operation of aircraft control surfaces. In Figs. 1A and 1B, an aileron 20 is operated by a hydraulic motor 22 under the control of a normally-closed valve 24 supplied with pressure liquid by a pump unit 23 comprising three constant delivery pumps 29, 30, 31 with low medium and high rates of delivery respectively. The valve 24 is operated from the pilot's lever 27, Fig. 1B, through a floating lever 26 also pivoted to the rod of the motor ram 21 so that movement of the latter automatically returns the valve 24 to its neutral position in which both the supply channels 39, 40 are cut off from the recess 25. The latter is supplied from a duct 35 fed from the pump delivery lines 32, 33, 34 past non-return valves 51, 49, 50 respectively. The lines 32, 33, 34 have byepass branches 36, 37, 38 which are connected with the interior of the valve 24 in its neutral position by ports 41, 42, 43, and are successively closed as the valve is moved away from the neutral position. The interior of the valve 24 is connecting through balancing ports 45 with a pipe 46 which discharges through a nozzle 47 into a sump 28. Fresh liquid is drawn in from a tank 48 by a pipe opening adjacent the outlet of the nozzle. The delivery lines 32, 33, 34 can also be connected with the pipe 46 by a spring-loaded relief valve 54 subject to the delivery pressure of the pump 29. A pipe 58 leads any leakage past the relief valve back to the tank 48. The ends of the motor 22 are selectively connected with the tank 48 through a pipe 66 containing a restriction 67 by a selector valve 59. This valve comprises a ported piston 60 moved by the higher of the pressures in the pipes 68, 69 to allow a restricted passage of fluid from the lower pressure pipe. The exhaust from the motor 22 is through pipes 71, 75 and ports 73, 76 in the valve 24. The pipes 71, 75 contain choke valves 72, 74 in the form of pistons 77 having lateral ports 79. The outer side of each piston is connected through pipe 80 or 81 with the supply channel 39 or 40 corresponding to the opposite side of the motor 22. These choke valves ensure that the motor 22 does not move quicker than the fluid supply warrants when the ailerons are being returned to their original position under aerodynamic loading. In Fig. 4 (not shown) these choke valves are combined in a single valve. Excessive pressures in the motor are relieved by valves 84, 85 connecting the two sides. A pipe 88 connects pressure-loaded flexible seals 86 at the. ends of the motor. The ram rod of the motor 22 also carries a damping piston 91 in a cylinder 90, the ends of which are connected by a restricted conduit 95 and are supplied with hydraulic fluid by an accumulator 93 past non- return valves 94. Normally, each aileron is operated by a separate hydraulic system. Fig. 1B shows the floating levers 26 of the two systems both connected to the pilot's lever 27. If the pressure supply fails, a normally-open pressure switch 97 closes and actuates a trip switch to open an electrically-actuated byepass valve 98 and disengage an electrically-operated clutch 99 so that the ram 21 can move freely and the pilot's lever 27 is without effect on the control until the trip-switch is reset. The valve 98 and the clutch 99 are similarly operated if the control valve 24 becomes jammed. In this event, a pre-loaded spring link 100 yields so that one of the switches 101, 102 closes and the trip-switch is operated. In the operation of this embodiment, the pumps 29, 30, 31 deliver successively into the duct 35 as the valve 24 is moved away from the neutral position. Due to the valves 49, 50, the pump 30 does not deliver until its pressure equals that of the pump 29 and similarly the pump 31 does not deliver until its pressure equals that of the pumps 29 and 30. As the valve 24 constantly tends to return to its neutral position, the amount of fluid delivered to the motor, which depends upon the degree of closing of the ports 41, 42, 43, will be governed by the rate of movement of the pilot's lever 27. Each aileron may be operated by two motors 22a, 22b, Fig. 5, either together or alternately should one fail. Each motor has a byepass valve 98a or 98b with an associated restriction 103a or 103b, a control valve 24a or 24b operated through a spring link 100a or 100b and a clutch 99a or 99b from a common operating member 27 and a pump unit 23a or 23b with a byepass valve 104a or 104b. If the motors are used alternately, the two systems A, B are under the control of a trip-switch which can be manually set to either of two positions. In one position, the motor 22a is operative, the byepass valves 98b and 104b are open and the clutch 99b is disengaged so that the motor 22b acts merely as a damping cylinder. If the pressure in the system A fails or its control valve 24a jams, the trip-switch is operated to cut out system A and bring in the system B. The change over from system A to system B or vice versa may be effected by manual operation. If both motors are operated simultaneously, the byepass valves and the clutch 99 of the system which fails or of which the control valve jams are operated to put that system out of action. The normally-open valves and the individual pumps are not shown in Fig. 5 which is diagrammatic. In Fig. 6, the control valve 24 is separated from the normally - open valves which regulate the amount of pressure liquid in the pump circuits. The latter are formed as a single valve 106 having three recesses 107, 108, 109 adapted to be moved successively out of register with the branches 36, 37, 38 of the delivery pipes of the low, medium and high delivery pumps 29, 30, 31 to bring the latter in turn into effective action. The valve 106 is subject at one end to delivery pressure of the pump 29 and at the other to the higher pressure prevailing in the motor 22 and to a spring pressure slightly greater than the pressure drop across the ports of the valve 24 when fully open and passing the maximum pump delivery. A shuttle selector valve 159 passes the higher motor pressure to the valve 106 and the lower motor pressure through a restriction 67 to the tank 48 and also to a choke valve 110. The latter is ,also subject to the pressure in the discharge pipe 46 and to the pressure of a spring and acts to restrict the movement of the motor under the aerodynamic load on the aileron so that no cavitation occurs. A relief valve 54 is provided as in Fig. 1A. In another modification, Fig. 7 (not shown), the spill valve 106 is replaced by three separate valves, each subject to delivery pressure of the pump 29 and to the higher pressure prevailing in the motor. Each of these valves is also loaded by a spring. The spring load of the spill valve corresponding to the high delivery pump is slightly greater than the pressure drop across the ports of the control valve when fully open and passing the maximum pump delivery whilst the spring load of the spill valve corresponding to the medium delivery pump is 50 lbs. per square inch greater. than the load on the first spill valve and the spring load of the remaining spill valve is 50 lbs. per square inch greater still. In Fig.. 8, a throttle valve 116 controls the spill from the pump unit 23 and is integral with two selector valves 117 which place the ends of the valve 116 selectively in engagement with the pump unit 23 and with an auxiliary constant pressure pump 119. The throttle valve 116 is connected to two pistons 120, 121 which are exposed to the pressures on the opposite sides of the motor 22 respectively. The delivery pressure of the pump 119 is selected to be twice the pressure on the exhaust side of the motor and the latter has a constant valve equal to the pressure drop across the ports of the control valve 24 when the latter are fully open and passing the maximum delivery of the pump unit 23. If the pressure on the working side of the motor increases due to the load on the aileron, this acts on one of the pistons 120, 121 to close the throttle valve more or less and so increase the pump delivery pressure. The latter acts in a contrary sense on the throttle valve to restore the balance. Only one pump of the unit 23 and therefore only one valve unit is shown.